Electrochromic cells of various designs have been proposed for use in such diverse apparatus as graphics displays, wrist watch displays, variable transmission windows and mirrors, and voltage or state-of-charge indicators for batteries, among others. Although their specific constructions may vary considerably depending on their intended application, most electrochromic devices possess several common characteristic features. Typically, an electrochromic device is comprised of an electrochemical cell having a first electrically conductive electrode which functions as cathode, a second electrically conductive electrode which functions as an anode, and a medium through which the first and second electrodes communicate to effectuate a visible change in the cell under the application of a DC voltage applied to a circuit including the first and second electrodes. The medium through which the electrodes communicate normally assumes the form of solid, semi-solid or liquid electrolyte. Any of the electrodes and the electrolyte may comprise an electrochromic material which responds to the current induced by the applied DC voltage so as to produce the desired visible change in the cell.
The operation of an electrochemical cell may produce permanent or reversible ionization or deionization of metals contained in either the electrolyte and/or at least one of the electrodes which results in an oxidation or reduction (redox) reaction at the respective electrodes. The cell is usually a confined space wherein the first and second electrodes are separated from one another by the electrolyte, possibly in conjunction with a spacing means. In addition, the cell normally includes at least one substrate, which may be fabricated from flexible or rigid material, through which the metallic deposition or dissolution occurring within the cell may be observed. For example, the visible change in the cell may be observed through the substrate if the substrate is substantially transparent or if the substrate contains an orifice or window through which the visible change may be observed.
Examples of electrochemical devices defined by enclosed cells with rigid substrates and a liquid electrolyte in communication with first and second electrodes include U.S. Pat. Nos. 4,116,535, 4,188,095, 4,285,575 and 4,902,108.
U.S. Pat. No. 5,411,817 discloses a charge indicator for a battery, in which a layer, between an electrode pair, comprises an antioxidant, a photographic color coupler dispersion and a photographic color developer. Various possible compositions for this layer are disclosed, but within the constraints just described, no disclosure appears to be provided to the effect that the electrochemical process, in which electrolyte color is transformed, is reversible without reversing the polarity of the electrodes. The other alternative applications disclosed, as a disposable electric meter and a "phone card," appear to imply that the electrochemical process is not reversible.
U.S. Pat. No. 3,720,869 discloses a method of determining a state-of-charge condition of an electrode in an electric cell by measuring the resistance of a cell electrode by its conversion between a metallic and non-metallic phase.
U.S. Pat. No. 3,667,039 discloses measuring instruments utilizing liquid crystalline elements that exhibit visible change in response to input signals above given threshold levels.
Several U.S. patents to Warszawski disclose light modulating devices as well as processes for making or using the same. These are U.S. Pat. Nos. 5,054,894, 5,056,899, 5,074,648, 5,078,480, 5,080,470 and 5,082,355. The substantive disclosures of all six of these patents are virtually identical. Thus, for convenience, these patents will be collectively referred to as the "Warszawski patents."
The Warszawski patents disclose numerous embodiments in the way of realizing several different types of light modulating devices, e.g., in large-scale environments, such as large signs in public locales. These patents emphasize maintaining the structural integrity of the light modulating devices disclosed, particularly in the case of large-scale devices, such that the electrolyte-electrode interfaces will not be unduly affected by local discontinuities and/or stresses.
To this end, the Warszawski patents propose the use of electrolyte layers, between opposing electrodes, that maintain a high degree of flexibility, and it is specifically proposed that the electrolyte material be plastic or viscoelastic. Also for the purpose of maintaining structural cohesion, it is proposed that the electrolyte layers either be provided with a separate adhesive or be self-adhesive themselves, to ensure adequate bonding with the electrodes.
Although the Warszawski patents appear to contemplate the use of electrodes that are not necessarily flat, there appears to be no teaching or suggestion to the effect of rendering the electrodes themselves as flexible. Thus, it would appear the Warszawski patents, while contemplating the use of flexible electrolyte layers, assume that the electrodes flanking the electrolyte layer(s) will essentially be rigid. This would appear to be particularly true in the case of large-scale public signs.
With regard to the manufacture of the light modulating devices disclosed in the Warszawski patents, different techniques are proposed, such as thick-film application techniques and cut-out techniques. The thick-film application techniques involve silk screening, air gap, helical wire bar and related techniques. The cut-out techniques involve, for example, the production of a composite, that includes an electrode with the electrolyte, and then applying the composite to another electrode. Contemplated are techniques such as extrusion, rolling, calendering, coating and the like, as well as punching, stamping and laser cutting.
U.S. Pat. No. 4,497,881 describes a battery charge indicator that is essentially formed from components already present within the battery itself. Particularly, there is disposed within the battery a charge producing compound which apparently could constitute the electrolyte, the anode gel, the cathode material or a combination of two or more of such compounds. This charge producing compound is provided just within the outer cylindrical surface of the battery housing and is configured to adopt a given color, such as black, when the battery has a full charge and gradually changes to a different color, such as gray, when the battery charge is depleted. A small hole or window is provided in the outer surface of the battery housing in order to enable the user to view the color of the charge producing compound. This patent appears to emphasize the use of materials already present in a conventional battery, rather than adding additional compounds or structures for indicating color.
U.S. Pat. No. 4,917,973 discloses a secondary battery in which one of the electrodes of the battery itself changes color in accordance with the voltage between the positive and negative electrodes of the battery. A window or hole is provided for viewing the color change of the aforementioned electrode. This patent appears to be limited solely to secondary batteries, i.e. significantly small, generally disk-shaped batteries.
U.S. Pat. No. 5,256,500 teaches a battery, such as a lithium battery, having a built-in lifetime indicator. Primarily, this patent is directed to lithium batteries, but also contemplates that the invention disclosed therein be utilized in conjunction with other types of batteries, such as manganese batteries and nickel cadmium batteries. It is generally suggested that an "indication element", as part of a "lifetime indicator" be provided within the battery itself. Various types of indicators are contemplated, such as electrochromism elements, electrophonetic cells and liquid crystal cells. In one embodiment, a "lifetime indicator" can be selectively mountable and dismountable with respect to a positive pull container of the battery. In this manner, the lifetime indicator can be mounted onto the battery only when it is desired to take a reading. For this embodiment, it appears that such a lifetime indicator be selectively mountable and dismountable via threaded engagements with the aforementioned positive pull container. No other possible embodiments of a removable lifetime indicator are disclosed.
U.S. Pat. Nos. 5,250,905, 5,396,177, 5,339,024, 5,418,086 and published PCT Application No. PCT/US92/07757 (International Publication No. WO 93/06474) variously describe batteries having externally mounted electrochemical tester devices. The tester devices themselves include the essential electrochemical cell components of a cathodic electrode, an electrolyte and an anodic electrode arranged in a label applied directly to the housing of a battery. In each of these references, the tester device is connected in constant parallel relation with the battery's terminals so as to provide a continuous reading of the battery's state of charge. The state of charge is determined as a function of the depletion or dissolution of the anodic electrode under the influence of the applied DC voltage. The testers also include an electrolyte which may be either a solid or a porous polymeric film matrix containing electrolyte solution. If solid, the electrolyte may be too brittle to withstand the externally applied forces and mechanical shock encountered by a battery under normal shipping, handling and usage. In these circumstances, the solid electrolyte may fracture, thereby severing communication between the electrodes and disabling the tester. Alternatively, when formed as a film matrix, the electrolyte must be preformed and thereafter applied to an electrode in a separate and distinct manufacturing step, thereby complicating and lengthening the label manufacturing process as well as adding to its expense.
U.S. Pat. Nos. 5,418,085 and 5,494,496 describe electrochemical battery tester devices which may be integrally affixed to an end of a battery. These references generally discuss electrochromic materials and that such materials may be printed on a substrate but offer no specific teachings of the details of how these materials may be printed under actual manufacturing conditions to produce an operable device. More particularly, there is no substantive discussion of the preferred compositions of the electrochromic materials and how these materials are selected and/or adapted to reliably perform under the rigors of normal manufacturing, shipping handling and usage of the battery. Nor is there any disclosure of whether such materials may be effective when borne by a tester label surrounding the circumference of a battery, which region is typically subject to greater and more frequent mechanical shock then the ends of a battery.
U.S. Pat. No. 5,458,992 teaches an electrochromic battery tester device that may be integrated into a battery label affixable to the circumferential wall of a battery housing. The electrolyte component of the electrochromic tester disclosed therein may be a solid or a thickened solution. If solid, the electrolyte may experience the aforementioned fracture failure under normal manufacturing, shipping, handling and usage of the label and battery. If formed as a thickened solution, there is no assurance that the solution may maintain its electrolytic properties throughout the useful service life of the battery, especially if the solution dries and the continuous presence of a threshold level of moisture is necessary for preserving electrolytic activity in the electrolyte and concomitant viability of the electrochromic cell. General references are made to methods of assembling the electrochromic cell by printing a label substrate using conductive and electrochemically active inks or paints. There is no disclosure however, apart from identification of certain active materials and thickeners, of specific ways or formulations by which such materials may be combined to produce viable inks or paints suitable for high speed printing.
An advantage exists, therefore, for an apparatus and method for producing, at high speeds, an inexpensive, thin film electrochromic cell which may be constructed as a series of preferably non-preformed layers applied to one or more flexible or rigid substrates, wherein each of the several layers may be deposited by coating or printing apparatus, and wherein the layers, including any electrode and electrolyte layers, maintain their as-applied structural and functional characteristics throughout the useful service life of the electrochromic device with which the cell may be used.
Although state-of-the-art high speed printing and coating techniques would appear to provide an apparent means for producing electrochromic cells, significant problems must be overcome in constructing electrochromic cells by such means. For example, the composition of each graphic and, particularly, functional ink used to construct the cell is critical because the inks' compositions dictate their electrical, chemical and mechanical properties and printability characteristics, and also because ink solvents occasionally tend to react detrimentally with the substrate materials to which the inks are applied. Because of these difficulties, conventional printing and coating products, processes and apparatus have not heretofore been used to assemble a functional and reliable electrochromic cell.
Accordingly, there exists a need for a versatile, high-speed, economical process and apparatus for making thin film electrochromic cells for a variety of devices. There also exists a need for processes, apparatus and electrochromic cell constituent materials which produce an electrochromic cell and/or device with indicia bearing surfaces to which permanent or temporary color graphics may be applied without interfering with the functionality of the electrochromic cell.